Well, you have to tick off the supernatural types you don't want to see in the game's "Options" before you start the new city. Then you won't have any supernatural townies at all, but you can still play them yourself.
"Toadify" is what a witch can do to you. However, most of the witches you don't play yourself are rather inept at raising their spellcasting level, which means that is mostly something that can happen in towns like Moonlight Falls, where you have pre-made level 8 or 6 witches. Elsewhere, many of the random townie "witches" never reach level 1. Aside from the easy to acquire "Cure" potion, a kiss from a lover can remove this.
Don't forget to have your sims take the "university aptitude test" before they go. The money usually doesn't matter, unless you are completely broke, but the credits you can get towards your degree can cut down the university time considerably.
